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  <h1><a href="http://freetype.org/index.html">FreeType</a>
    Design&nbsp;/&nbsp;V</h1>
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          <h2>V. Interfaces and Services</h2>

          <p>We shall now go into detail about interfaces and services
            in FreeType.</p>

          <p><em>Interfaces</em> in FreeType are analogous to those
            found in object-oriented programming.  They can be thought
            of as internal public APIs, and are essentially tables of
            function pointers.</p>

          <p>Interfaces only describe the form and functionality, but
            the actual function body may be implemented elsewhere.
            The module that is implementing the interface will then
            pass the required function pointers to the table.  This
            gives modularity and easy extendability.</p>

          <p>There are two main kinds of interfaces: <em>module</em>
            interfaces, and <em>services</em>.</p>

          <p>Module interfaces are defined for each module.  For
            example, every font driver provides its own set of
            procedures for use in the base layer, which are registered
            in an <tt>FT_Driver</tt>.  This way, very different font
            drivers can be used in the same way in the base layer.</p>

          <p>Services are cross-module interfaces.  These provide
            functionality needed in several font drivers. </p>

          <p>Services are created when code from one module needs to
            be used in another.  Rather than include files from
            another module, a service is created instead.  Now, the
            other module just needs to include the header defining the
            interface.</p>

          <p>Helper modules are an extreme example of this; all their
            public functionality is made for use in other font drivers
            and hence are in a single service.</p>


          <h3>In-depth guide: Creating a service</h3>

          <p>This section will teach you how to write your own
            service.</p>

          <ol>
            <li>
              <p>Make the service interface header.</p>

              <p>We will be calling our service demo for demonstration
                purposes.  First, create the header file, which goes
                in <tt>include/freetype/internal/services</tt>, and
                add the boilerplate.</p>

              <pre>
#include FT_INTERNAL_SERVICE_H
FT_BEGIN_HEADER
#define FT_SERVICE_ID_DEMO  "demo"

/* ...  */

FT_END_HEADER</pre>

              <p>This line in particular is required to register the
                new service later on.</p>

              <pre>
#define <em>service-id service-tag</em></pre>

            </li>
            <li>
              <p>We will have identified some functions that are
                needed in another module.  Extract the function
                signatures of these and place them in the header.</p>

              <pre>
[typedef <em>return-type</em>
   (*<em>type-name</em>)(<em>function-signature</em>);]+</pre>

              <p>Example:</p>

              <pre>
typedef FT_Error
  (*SampleDoSomethingFunc)( int  foo );
typedef void
  (*SampleDoAnotherFunc)( int    foo,
                          float  bar );</pre>

            </li>
            <li>
              <p>Define the service interface.</p>

              <p>Use the <tt>FT_DEFINE_SERVICE</tt> macro to do
                this.</p>

              <pre>
FT_DEFINE_SERVICE( <em>service-name</em> )
{
  [<em>type-name  interface-entry</em>;]+
}</pre>

              <p>Example:</p>

              <pre>
FT_DEFINE_SERVICE( Demo )
{
  SampleDoSomethingFunc  doSomething;
  SampleDoAnotherFunc    doAnother;
};</pre>

              <p>Here is the definition of the above macro
                (in <tt>ftserv.h</tt>).</p>

              <pre>
#define FT_DEFINE_SERVICE( name )            \
  typedef struct FT_Service_ ## name ## Rec_ \
    FT_Service_ ## name ## Rec ;             \
  typedef struct FT_Service_ ## name ## Rec_ \
    const * FT_Service_ ## name ;            \
  struct FT_Service_ ## name ## Rec_</pre>

              <p>This defines a new struct
                called <tt>FT_Service_DemoRec</tt>, along with a
                handle type for the
                struct, <tt>FT_Service_Demo</tt>.</p>
            </li>
            <li>
              <p>Register (and/or implement) the functions for the
              newly created interface.</p>

              <p>In the module implementing the interface, add a
                definition like the following</p>

              <pre>
static const <em>service-record  table-name</em> =
{
  <em>function-name</em>
  [,<em>function-name2</em> [,...]]
}</pre>

              <p>which corresponds to the interface defined in
                step&nbsp;3.  Example:</p>

              <pre>
static const FT_Service_DemoRec  demo_service_rec =
{
  function1,
  function2
};</pre>

              <p>This initializes the function pointers table.  In
                this example, <tt>function1</tt> has the function
                signature of <tt>SampleDoSomethingFunc</tt> and
                implements the <tt>doSomething</tt> functionality, and
                so on.</p>
            </li>
            <li>
              <p>Register the new service.</p>

              <p>Next, add this code to define the service list in
                this module, or append the new service to an existing
                list.</p>

              <pre>
static const FT_ServiceDescRec <em>service-list-name</em>[] =
{
  { <em>service-id</em>, &amp;<em>table-name</em> },
  [{ <em>service-id2</em>, &amp;<em>table-name2</em> }, [...]]
  { NULL, NULL }
}</pre>

              <p>Example:</p>

              <pre>
static const FT_ServiceDescRec demo_services[] =
{
  { FT_SERVICE_ID_DEMO, &amp;demo_service_rec },
  { NULL, NULL }
};</pre>

              <p><em>service-id</em> is what we <tt>#define</tt>'d in
                the service header file in step&nbsp;1. Note that
                the <tt>{NULL,&nbsp;NULL}</tt> sentinel value at the
                end is required.</p>

              <p>Now we need a way for other modules to find this
                service.  First, we need to implement
                the <tt>get_interface</tt> function in
                <tt>FT_Module_Class</tt>.  Here is a minimal example
                that does not do any validation.</p>

              <pre>
FT_CALLBACK_DEF( FT_Module_Interface )
<em>get-interface-name</em>( FT_Module    module,
                    const char*  module_interface )
{
  return ft_service_list_lookup( <em>service-list-name</em>,
                                 module_interface );
}</pre>

              <p>Then, pass it into the <tt>FT_DEFINE_MODULE</tt>
                macro for this module.</p>

              <pre>
(FT_Module_Requester) <em>get-interface-name</em></pre>

              <p>The last step is optional but recommended, which is
                to register the new service header
                in <tt>ftserv.h</tt>.

                <pre>
#define FT_SERVICE_DEMO_H  &lt;freetype/internal/services/svdemo.h&gt;</pre>

            </li>
            <li>
              <p>Use the new service.</p>

              <p>Now, in the file that wants to use the service, add
                the following code to get the service.</p>

              <pre>
<em>service-record-handle</em>  service;


FT_FACE_FIND_GLOBAL_SERVICE( face,
                             service,
                             <em>service-id-tail</em> );</pre>

              <p><tt>face</tt> should be of type <tt>FT_Face</tt>,
                which is usually the current face instance being used
                in the driver, and FreeType tries to find the service
                in the driver of this face first, before searching all
                other modules.</p>

              <p><em>service-id-tail</em> is the part
                of <em>service-id</em>
                following <tt>FT_SERVICE_ID_</tt>.

              <p>Now to call some function in the service.</p>

              <pre>
service-><em>interface-entry</em>( <em>params</em> );</pre>

              <p>Example:</p>

              <pre>
FT_Service_Demo  demo;
FT_Error         error;


FT_FACE_FIND_GLOBAL_SERVICE( face, demo, DEMO );

error = demo->doSomething(0);
</pre>

            </li>
          </ol>
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        <div class="updated">
          <p>Last update: 13-Feb-2018</p>
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